In nuclear reactors, the transfer of heat between the primary fluid for cooling the core of the reactor and the vaporized water for driving the turbines used to produce electricity is effected by means of steam generators. Inside these steam generators, a bundle of long tubes ensures the heat transfer while avoiding any direct contact between the two fluids.
In certain types of steam generators, the bundle of tubes is placed in a cylindrical casing with a horizontal axis and the extremities of each of the tubes open into two cylindrical collectors with vertical axes and placed inside this casing. Access openings, formed for example at the upper extremities of the collectors, make it possible to gain access to the latter once a plug has been removed.
Because of the corrosive nature of the fluids used to cool nuclear reactors, it is necessary to periodically test the tubes of the bundle, which may be effected, for example, by sending a search gas into each of the tubes or by introducing into these an eddy current probe.
Having regard to the high radioactivity existing inside the collectors of the steam generators, it is preferable to be able to have a device making it possible to remotely position a member, such as a guide tube, through which an eddy current probe is able to be introduced successively opposite each of the perforations opening into this collector.
Such a device, which needs to be relatively easy to implement, must also take account of the fact that, although the perforations are disposed along a regular network, this network most often has discontinuities and some of the perforations have been sealed during a maintenance operation carried out previously. In addition, the spacing between the perforations may vary slightly, due to manufacturing tolerances.
So as to carry out such control operations, there currently exists a device, including a pole disposed along the vertical axis of the collector, which supports a unit designed in such a way as to inject an eddy current probe. Each of the perforations is reached by rotating the pole and by moving the injection unit vertically along this pole.
This known device has a certain number of drawbacks. In the first place, its placing needs to be effected with extreme precision and requires the presence of an additional centering star at the lower extremity of the pole. In addition, even if the device is placed correctly, the irregular nature of the network of perforations necessitates a long and delicate implementation procedure. Those personnel using this device therefore have to undergo special training. Furthermore, the whole device enabling the eddy current probe to be introduced and extracted is placed inside the collector, this not really being desirable, having regard to the radioactive contamination existing inside the latter.
There also exists a large number of devices designed to position a member, such as a guide tube with an eddy current probe, inside the collectors of steam generators with different structures. The tubes inside these collectors are connected onto a horizontal plate with tubes, the collectors being under the plate. A description of these existing devices is contained in the documents EP-A-O 030 484 and FR-A-2 382 986.
In addition, the document FR-A-2 382 986 describes a device having a vertical column taking support respectively on a tube plate and on the wall of the steam generator situated opposite the latter. The column supports a horizontal arrow on which a carriage is able to move, the carriage bearing the member which is to be positioned opposite the tubes. The positioning is obtained by combining a displacement of the carriage along the arrow and a rotation of the column around its own axis.
These existing devices are specifically designed so as to be used in the collectors of steam generators delimited between a horizontal tube plate and a lower hemispherical wall of the steam generator. In no instance may they be transposed to cylindrical collectors provided with perforations opening radially inside these collectors unless they have been considerably modified.